Process of electroplating zinc, and baths and compositions for use therein



Patented Nov. 15, 1949 UNITED STATE PROCESS OF ELECTROPLATING zINo, ANDBATHS AND COMPOSITIONS FOR USE THEREIN Jesse, E. Stareck, Birmingham,and Frank lfassalacqua, Detroit, Mich., assignors to United Chromium,Incorporated, New York, N. Y., a

corporation of Delaware No Drawing. Application August 25, 1944,Serial'No. 551,258

This invention relates to the electrodeposition of zinc, and providesimprovements therein.

The invention provides a new and improved process for electrodepositingzinc,- and novel baths and compositions foruse in such process.

It is desirable and advantageous (l) to electroplate smooth zincdeposits, (2) to electroplate bright deposits of zinc, (3) toelectroplate such deposits with high current efiiciency, (4) to make useof noncyanide alkaline baths which are free from cyanide fumes, (5) tomake use of baths less alkaline than cyanide-zinc baths, yet of suchalkalinity that iron is not soluble in the bath, (6) to have baths inwhich the pH remains fairly stable, (7) to have an alkaline bath withwhich air or mechanical agitation can be used without building upcarbonates or other objectionable reaction products therein, (8) toelectrodeposit zinc at greater speed than by other processes making useof alkaline baths, (9) to avoid as much as possible hydrogenembrittlement of the base metal in the process of electrodepositing thezinc, (10) to obtain brighter deposits than are obtained by processesheretofore used, (11) to electrodeposit zinc on cast iron withoutspecial preliminary'steps, (12) to have a process of electrodepositingzinc which works well in barrel plating, (13) to make use of baths whichare not corrosive to equipment and which do not give off poisonousfumes; the foregoing advantages are obtained by the use of the processand baths of the present invention.

According to the present invention a bath is used which contains thecomplex compound potassium zinc pyrophosphate KeZn(P2O'1)2, having acontrolled pH, and containing a substance which acts as a brightener anda throwing power promotor. The pyrophosphate radicals and zinc contentare in the molecular ratio of 2 to 1 plus an excess of pyrophosphateradicals; with said excess the molecular weight ratio range is from 2.3to 1 to 4.6 to 1.

Compositions of water soluble zinc and pyrophosphate compounds may bemade up .for preparing baths with zinc and pyrophosphate radicalcontents as given above.

The bath may contain other substances which give known effects, such aswetting agents, antifeamants, buffering agents, anode depolarizers, e c.

For making up the bath, (and also thecomposition of matter) a zinccompound such as zinc sulphate or zinc chloride or zinc pyrophosphatesoluble in an aqueous solution of potassium or sodium pyrophosphate, andpotassium or sodium pyrophosphate, may be used. The sodium pyrophosphateis less soluble than the potassium pyrophosphate. As the commercialchemicals vary somewhat in composition, they should be .4 Claims. (01. 204-55) used on the basis of the ascertained pyrophosphate and zinccontent.

Examples of baths can be greater or less than those given in theexamples above; the concentrations given are'good workingconcentrations. p p

A brightener is added to the bath, and is a solution soluble carbocycliccompound having hydroxy and methoxy groups ortho to one another,specific examples of which are curcumin, phenol sulphonate condensationproducts of vanillin, and ortho-hydroxy meta-methoxy benzaldehyde, thelast of which we have foundto be the most satisfactory to use. Vanillinis also included in the above class, its usefulness, however, appearingto beinbaths having a pH of about 9.5.. Brighteners of this type, whenadded to the bath, act .as very eilicient hydrogen inhibitors duringplating, especially on ferrous base metals. Besides. producing abrightening action, the addition of brighteners will also markedlyincrease the operating current density range. Oxidation products ofthebrightener, as produced by electrolysis or the addition of anoxidizing agent as for example potassium permanganate, act'as promotorsof throwing power. j Thequantity of brightener used is small as regardsthe other bath constituents, and in the baths accordingto the examplesgiven above, the concentration is generally maintained at onehalf .tothree .gramsper liter depending on the brightener used and itssolubility. The brightener'should be added to the bath in amounts toproduce brightness under optimum working condition's. The. brightness ofthe Zinc deposit equals that or mirror-likechornium deposits with whichthe art is familiar. I I I The alkalinity, or pH, of the bath ismaintained between 9.5 and 12.5,.with the optimum between 11.2 and 12.0.The effect of maintaining the bath in the aforesaid pH range is toeliminate roughness of the deposit and to give completecoverage, withthe brightener present; high current efficiencies are also obtained. Itis an important and concomitant advantage of the invention that therange of pH or alkalinity of the bath is above the pH value at whichiron dis- The average current density may also vary considerably; withbaths of the composition given in the above examples, current densitiesup to 80 amperes per square foot may be used with good results. With theaforesaid baths, current densities of to 70 amperes per square foot areoptimum. The current efiiciency is nearly 100%, and the process of the.present invention, over other alkaline processes for electrodepositingzinc, has the advantage of causing much less hydrogen embrittlement.Moreover, the speed of deposition is considerably higher than with otheralkaline baths heretofore in use. The throwing power is good, beingbetter than that of acid-zinc processes, and generally better than thatobtained from cyanide-zinc processes.

It is desirableto agitate the baths in operation, and this may be doneeither by air or mechanical means. Neither carbonates nor otherobjectionable reaction products accumulate by air agitation. Ii foamingoccurs this can be eliminated by means of adding a suitable antifoamingagent.

' We have discovered a novel and very -advantageous antifoaming agentfor use in our baths in the substance tri-octylphosphate.

Foranodes, electrolytic zinc or cast zinc of high parity is recommended.Citrate radical in the bath, in quantity of about 15 g./l., may beused-as a depolarizer.

Care should be taken to exclude metallic impurities, especially lead,hexavalent chromium, copper, cadmium and arsenic. In general, the bathshouldbe kept pure.

It is desirable to electrolyze the baths before using for platingoperations, or to add a small quantity of potassium permanganate tobuild up or promote the throwin power of the bath.

A mode of procedure followed in the plating of steelcartridge cases isas follows:

Bath composition The steel cartridge cases were put on a rack, im-

mersed in the bath, and connected to the cathode bus-bar. Current waspassed for three minutes 'ata current density of 60 amperes per squarefoot, at the end of which time there was obtained a uniform, smooth,mirror-bright deposit 0.0002 inch thick. The temperature of the bath was115 F. and air agitation was used. The anodes were cast electrolyticzinc.

The baths and process of this invention can be satisfactorily used forbarrel-plating.

'The zinc deposits obtained by the use of the present invention aresmooth, and smooth and bright under optimum operating conditions, thebrightness being like that of mirror-like chromium deposits. Thedeposits have a fine grained crystalline structure typical of trulybright metallic deposits.

What is claimed is:

1. An aqueous bath for electroplating zinc, con-.

sisting essentially of pyrophosphate radicals and .zinc in the molecularratio of approximately 2.3 to-l to 4.6 to 1, and optimum amounts ofapproximately /2 to 3 g./l. of ortho-hydroxy meta methoxy benzaldehyde,acting as a brightener, and having a pH between 9.5.and 12.5.

2. An aqueous bath for electroplating zinc, according to claim -1,wherein the pH is between 11.2 and;12.0.

, 3. A'p'rocess of electrodepositing smooth zinc plate, which comprisespassing an electric current from an outside source'from an anode to-acathode-article immersed in an aqueous bath solution at room temperatureto 140 F. and at 1 to amperes per square foot consisting essentially olpyrophosphate radicals and zinc in the molecular ratio of approximately2.3 to 1 to 4.6 to

1, and optimum amounts of approximately to 3 grams per liter ofortho-hydroxy meta methoxy benzaldehyde, acting as a brightener,.andhaving a pH between 9.5 and 12.5.

4. A composition of matter, consisting essentially of an alkali-metalpyrophosphate, zinc pyrophosphate which is soluble in a water-solw tionof the alkali-metal pyrophosphate, and ortho-hydroxy meta methoxybenzaldehyde as a brightener, the ratio of the pyrophosphate radi- .calsto the zinc in the aforesaid ingredients being in the molecular weightratio range.2.3 tolto 4.6 to 1, and the amount of the brightener beingto 3 parts brightener for 250 parts zinc and pyrophosphate radicals inthe composition.

JESSE E. STARECK. FRANK PASSALACQUA.

REFERENCES CITED The following references are of record in the file .ofthis patent:

UNITED STATES PATENTS,

OTHER REFERENCES Ser. No. 355,522, Howaldtet al. (A. P. C.) pub, May18,1943. Journal of American Chemical Society (18) Electrodeposition ofMetals, by Langbein and Braunt, 9th edition, 1924, page 567.

Wood Chemistry, by Wise, 1944 edition, pages Transactions of theElectrochemical Society, vol. 88 (1945), page 272.

